Laser device with pendulum-type pressure roller

ABSTRACT

The invention concerns a laser device for joining work pieces made of plastic by means of beam welding technology, with a processing head that has focusing devices for the laser beam. Here, the focusing device that is closest to the work pieces has a rotatably supported cylinder lens in the shape of a roller that focuses the laser beam along a single axis and presses the work pieces together in the form of a line in an area around the impact point of the laser beam. In order to enable the cylinder lens to always contact the surface of the work piece facing the processing head in a precisely tangential manner, the processing head has a pendulum-type suspension for the rotatable cylinder lens. This also makes it possible for the contact pressure exerted by the processing head to always act perpendicularly on the work pieces so that they are pressed together in all welding places in the same way in a defined manner during the welding process.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority under 35 USC §119 to GermanPatent Application No. 20 2008 006 180.6, filed on May 6, 2008, theentire disclosure of which is incorporated herein by reference.

1. Technical Field of the Invention

The invention concerns a laser device for joining work pieces made ofplastic by means of beam welding technology, with a processing head thathas focusing devices for the laser beam. Here, the focusing device thatis closest to the work pieces has a rotatably supported cylinder lens inthe shape of a roller that focuses the laser beam along a single axisand presses the work pieces together in form of a line in an area aroundthe impact point of the laser beam.

2. Description of the Related Art

In beam welding technology, adjacent contact surfaces of the work piecesto be joined are melted by the effect of a laser beam and are joinedtogether under pressure during the subsequent cooling phase. This methodof plastic welding by means of a laser beam is commonly known. In thiswelding process, it is important that the work pieces are clamped toeach other during the welding process because not just the energy dosagebut also the direct mechanical contact between the surfaces to be joinedis an important prerequisite for a good welding joint. In the field,laser devices of many different designs are known.

With three-dimensional welding contours in particular, the clamping ofthe work pieces with conventional clamping devices requires considerableefforts if sufficient contact pressure is to be exerted on the workpieces over the entire welding contour at the same time. Prior artprovides for a robot-controlled processing head to simultaneously guidethe laser beam to the spot to be joined and to press the work piecestogether mechanically.

EP 1 405 713 B1 discloses a laser device where the work pieces arepressed together either point-by-point at the precise spot where thelaser beam impacts or also over a wider area in a region around theimpact spot of the beam or along a line perpendicular to the relativemovement of the laser beam. This offers the possibility for applying therequired clamping force simultaneous with the laser beam along aprogrammable welding contour in a continuous but always point-by-pointprocess along a line or in a certain area to the very spot at which thewelding process is taking place and where the clamping force is actuallyneeded. This is made possible by a processing head that can be moved inthree dimensions, that can be moved at an even distance from the workpieces, and that assumes the transmission of a mechanical contactpressure as well as of the laser energy.

The relative motion between the processing head and the work pieces tobe welded together is always carried out under mechanical contact whilethe work pieces are being pressed together at the impact point of thelaser beam, among other methods preferably by a roller that istransparent for infrared radiation. The pressure roller, made of glassor plastic for example, is part of the focusing device of the processinghead for the laser beam. Thus, it has the optical characteristics of acylinder lens that focuses the beam in the form of a line along a singleaxis onto the contact surfaces of the work pieces. The cylinder lensacting as a pressure roller is rotatably attached to the processing headin order to prevent unnecessary mechanical friction. When following thewelding contour, the cylinder lens is moved with a rolling motion overthe surface of the work pieces in the course of which the laser beam canimpact as a line on the work pieces in the forward feeding direction orperpendicular to the forward feeding direction.

In the known laser device, the cylinder lens pressing the work piecestogether is centrically rotatably supported at both face surfaces, withthe position of the lens relative to the processing head beingunchangeable. In relation to the longitudinal axis of the processinghead, and therefore also in relation to the optical axis of the focusingdevices, the center axis of the cylinder lens is fixed, preferablyforming an angle of 90 degrees therewith. In order for the contactpressure as well as the laser radiation to have an optimal effect on thework pieces, it is necessary to guide the processing head at a givenspeed along the welding contour while always providing for itsperpendicular alignment relative to the spot that is being welded. Forthis purpose, the movement of the processing head is preciselycontrolled by a robot system that determines the speed of motion, thepath of the motion, and the alignment of the head relative to the workpieces in accordance with a programmed specification. In case ofslightly deformed three-dimensional work pieces, or in case of minoruneven areas on the work pieces, the programmed attitude of theprocessing head and therefore of the cylinder lens providing pressuremay deviate from its ideal position relative to the work pieces, therebyreducing the quality of the resulting welding seam.

With reference to this prior art, the invention addresses the problem ofproposing a solution that would ensure a uniform contact pressure on thework pieces at all times.

SUMMARY OF THE INVENTION

According to the invention, the processing head has a pendulum-typesuspension for the rotatable cylinder lens. This enables the cylinderlens to always contact the surface of the work piece facing theprocessing head in a precisely tangential manner. Thereby, the contactpressure exerted by the processing head always acts perpendicularly onthe work pieces so that they are pressed together in all places in thesame way in a defined manner.

If the attitude of the processing head is not absolutely perpendicularin relation to the work pieces, the cylinder lens is automaticallypivoted around its center as soon as it contacts the work pieces. Thisenables the cylinder lens to automatically alter its alignment relativeto the processing head, with the nature of the connection permitting nomovement of the virtual axis of rotation of the cylinder lens in thedirection of motion of the processing head, but allowing only analignment in the direction of the longitudinal axis of the processinghead. Thus, the cylinder lens can only tilt around an axis that isperpendicular to the direction of motion of the processing head.

Preferably, the type of the connection is selected so that the axis oftilt of the pendulum-type suspension passes through the center of thecylinder lens. As a result, the laser beam passes through the cylinderlens always centrically, even if the cylinder lens tilts, so that thefocus line generated by a tilted lens on the work pieces is also notchanged significantly.

In a preferred embodiment of the invention, the cylinder lens acting asa pressure roller is connected to the processing head by means of auniversal joint. This is accomplished by attaching a central crosselement at two opposite ends with rotatable joints to the processinghead and to the cylinder lens.

Technically, this type of Cardanic suspension can be implemented bymeans of a metal ring that, on the one hand, is tiltably connected tothe processing head via two first axles extending in the direction ofmotion of the processing head and, on the other hand, is connected tothe cylinder lens via two additional second axles. The cylinder lens islocated in the cavity enclosed by the metal ring, with the second axlesextending perpendicular to the direction of motion of the processinghead. The second axles that support the cylinder lens in an axiallyrotatable manner are offset by 90 degrees relative to the first axles.

In a practical variant of the invention, the cylinder lens is connectedto the pendulum-type suspension at only one face surface. This makes itpossible to guide the cylinder lens closer to edges or corners of workpieces, thereby simplifying the welding process in such places. In theembodiment described above, this can be accomplished in simple fashionby using a ring segment instead of a full metallic ring, with the ringsegment extending over a segment angle of 270 degrees and determining atypical gap of 90 degrees. Here, the gap of the ring segment isdiametrically opposed to the side of the cylinder lens that is connectedto the ring segment.

Below, the invention is explained in detail with reference to anembodiment shown in the drawing. Additional characteristics of theinvention may result from the following description of the embodiment ofthe invention in conjunction with the claims and the attached drawing.The individual characteristics may be implemented either independentlyor in combinations of several in different embodiments of the invention.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described andparticularly pointed out in the claims. The following description andthe annexed drawings set forth in detail certain illustrativeembodiments of the invention. These embodiments are indicative, however,of but a few of the various ways in which the principles of theinvention may be employed. Other objects, advantages and novel featuresof the invention will become apparent from the following detaileddescription of the invention when considered in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side view of a holding tube of the processing head of alaser device according to the invention for a cylinder lens that pressesthe work pieces together and focuses the laser beam;

FIG. 2 shows the holding tube from FIG. 1 in a front view, with a viewof the cylinder lens;

FIG. 3 shows the holding tube from FIG. 1 in a first axial section view,perpendicular to the center axis of the cylinder lens; and

FIG. 4 shows the holding tube from FIG. 1 in a second axial sectionview, perpendicular to the longitudinal axis of the cylinder lens.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a holding tube 1 of a processing head (not shown in thedrawing) of a laser device according to the invention. The holding tube1 has the form of a stepped hollow cylinder and can be attachednon-permanently by means of an attachment device 2 to the processinghead after it is inserted with an upper connection section 3 into acavity of the processing head. In addition, focusing devices areprovided in the processing head that shape and guide an entering laserbeam arriving from a laser source.

At a lower attachment section 4 of the holding tube 1, said attachmentsection following directly, with a larger diameter, after the connectionsection 3, a cylinder lens 5 is connected movably with the holding tube1 via a receiving ring 6. The holding tube 1 and the receiving ring 6are made of metal. The cylinder lens 5 is permeable for the laser beam(not shown in the drawing) and consists of glass or a transparentplastic material.

FIG. 1 and FIG. 2 show the cylinder lens 5 in a tilted position relativeto the holding tube 1. In a normal position (not shown in the drawing),the receiving ring 6 and the cylinder lens 5 are aligned perpendicularto the axis of the holding tube 1.

FIG. 2 shows the holding tube 1 according to FIG. 1 in a front view,with a view of the cylinder lens 5. The cylinder lens 5 is mounted inthe holding tube 1 with a two-dimensional degree of freedom by means ofa Cardanic suspension. The receiving ring 6 is tiltably connected to theholding tube 1 by means of two first axles 7, 7′, of which the axle 7 isalso visible in FIG. 1. Offset by 90 degrees in relation to the firstaxles 7, 7′, two second axles 8, 8′ are provided that rotatably connectthe cylinder lens 5 to the receiving ring 6.

The axles 8, 8′ are in alignment with the axial center axis 9 of thecylinder lens 5. This allows the cylinder lens 5 to rotate in thedirection of motion of the processing head when it is pressed againstthe work pieces (not shown in the drawing) and when the processing headis moved forward or backward perpendicular to the center axis of thecylinder lens 5.

If, at any location of the welding contour, the processing head with theholding tube 1 is not aligned precisely perpendicular relative to thesurface of the work pieces, the receiving ring 6 carrying the cylinderlens 5 automatically aligns itself parallel and/or tangential inrelation to the surface of the work pieces due to the contact pressureacting on the cylinder lens 5 asymmetrically to the center 10 of thecylinder lens 5. In doing so, the receiving ring 6 tilts around theaxles 7, 7′.

FIG. 3 shows the holding tube 1 in a first axial section view where thecylinder lens 5 is sectioned in transverse direction perpendicular toits center axis 9. As in the other figures of the drawing, the receivingring 6 is tilted around the two axles 7, 7′ by several degrees of anglein relation to the holding tube 1. The center axis 9 of the cylinderlens 5 that runs in the longitudinal direction is located in theextension of the central longitudinal axis 11 of the holding tube 1along which the laser beam (not shown in the figure) also extends. As aconsequence, in the normal as well as the tilted position, the laserbeam always impacts the cylinder lens 5 in the area of the center planethat is determined by the center axis 9.

FIG. 4 shows the holding tube 1 in an axial section view that is offsetby 90 degrees compared with FIG. 3. The axles 8, 8′ for the cylinderlens 5 are held on both sides in friction bearings 12, 12′ that arepress-fitted into the receiving ring 6. The center 10 of the cylinderlens 5 is located in a direct extension of the longitudinal axis 11 ofthe holding tube 1 so that the laser beam passes centrally through thecylinder lens 5 regardless of the tilt of the cylinder lens 5 inrelation to the holding tube 1. As a consequence, the spatial locationof the focus line in relation to the work pieces is only influenced toan insignificant degree by the tilting of the cylinder lens 5.

Further features of the invention can be found in the followingdescription of preferred embodiments of the invention in connection withthe claims and the drawings. The single features can be realised aloneor several together in embodiments of the invention.

1. A laser device for joining work pieces made of plastic by means ofbeam welding technology, with a processing head that has focusingdevices for the laser beam, where the focusing device that is closest tothe work pieces has a rotatably supported cylinder lens in the shape ofa roller that focuses the laser beam along a single axis and thatpresses the work pieces together in the form of a line in an area aroundthe impact point of the laser beam, wherein the processing head has apendulum-type suspension for the rotatable cylinder lens.
 2. A laserdevice as claimed in claim 1, wherein the axis of tilt of thependulum-type suspension passes through the center of the cylinder lens.3. A laser device as claimed in claim 1, wherein the cylinder lensacting as pressure roller is connected to the processing head by meansof a universal joint.
 4. A laser device as claimed in claim 1, whereinthe cylinder lens is connected to the pendulum-type suspension with oneface surface only.
 5. A laser device as claimed in claim 2, wherein thecylinder lens acting as pressure roller is connected to the processinghead by means of a universal joint.